A conventional superheterodyne receiver (superhet) is based upon the mixing of a received signal with a local oscillator ("L.O.") signal to yield a new (intermediate frequency or "I.F.") signal which is the difference or sum of the L.O. and off-air signal frequencies. Tuning of the receiver is effected by varying the local oscillator frequency, thereby changing the frequency of the off air signal which when mixed with the L.O. signal, will yield a signal at a fixed I.F.
A disadvantage of the superheterodyne receiver is that received signals having a frequency higher or lower than the L.O. frequency and differing from the L.O. frequency by the I.F. either positively or negatively will mix to yield signals at the I.F. This is known as the reception of the "image" and may be partially overcome by the use of a higher intermediate frequency. This is often followed by a second conversion to a lower frequency where adjacent channel filtering is performed. Double superheterodyne receivers commonly have the first I.F. at 10.7 MHz so that the image frequency is 21.4 MHz away from the desired signal.
In order to save battery power, pager receivers "hibernate" or "sleep" for certain periods of time, turning themselves on regularly at predetermined intervals in order to check for addressing from a transmitter. One such form of time division multiplexed addressing is that specified in CCITT 584-1 paging format, also known as POCSAG. A disadvantage of this form of time division addressing is that power consumption is constant regardless of channel traffic. Another disadvantage is that if the receiver is required to receive large amounts of data to be transmitted during off peak hours it is necessary in current state of the art pagers to continuously operate, according to the paging code format.
Current state of the art radio pagers generally have their features and functions defined by firmware designed into the product, and most have a level of programmability, using special attachments, to provide for selection of alternate response characteristics from a small range of options offered. A disadvantage of this is that all of the receiver response characteristics have to be defined at the time of design and the receiver cannot be modified to meet the requirements of a specific user or service provider.
Current state of the art radio pagers generally include vibrators within the body of the pager so that the user may be silently alerted to the receipt of a page. For maximum effectiveness, it is necessary for such device to be firmly coupled to the wearer, as is the case when worn on a belt. The disadvantage of this is that when the pager is not so carried, the vibrator has limited effectiveness and an alternative form of alert is desirable.
Radio paging receivers commonly use replaceable Alkaline batteries or rechargable batteries of the same form interchangeably. The detection of low battery reserve is generally determined by evaluation of the terminal battery voltage under load. A disadvantage of this is that disposable battery discharge characteristics differ from those of rechargeable batteries and accurate detection of low battery reserve for both types using a common detection method is generally only accurate for one type.
AFC (Automatic Frequency Control) is very desirable on UHF (Ultra High Frequency) receivers operating above 500 MHz. At these frequencies the frequency errors in the crystal oscillators used to generate the local oscillator signals in the receiver amount to a few parts per million, and are comparable with, and can exceed, the frequency tolerance of the receiver (for maximum sensitivity). These errors result from sensitivity to temperature and from long term frequency drift caused by aging of the crystal.
AFC is widely used on analog systems. For such receivers on channel center, the integral of the deviation over a period of a few hundred milliseconds or less is zero, and because transmission (and reception) is continuous, capacitors can be used to integrate and store the AFC feed back signal.
Simple analog systems cannot be used in receivers which are strobed (for channel sampling) or which are designed to be used with digital, time dependent addressing techniques (such as CCITT 584-1). There are three principal difficulties:
a) In digital systems, the FSK (Frequency Shift Keyed) modulated carrier may contain very low frequency components due to long streams of 1's or 0's.
b) The periods between samples can be as long as one second so that analog techniques to store feedback signals (such as sample and hold) are marginal and error prone.
c) To minimize power consumption, the receiver should be prealigned to minimize the period time required by the receiver to sample the channel.